Measuring blood pressure is an important diagnostic tool in many medical treatments, especially when treating vascular maladies. For example, aneurysms are often treated by implanting a stent-graft within the aneurysm pocket. Monitoring blood pressure at the stent-graft can be important in tracking patient health and treatment effectiveness. Various pressure sensors have been used for monitoring blood pressure within a vessel, including capacitive pressure sensors. These capacitive pressure sensors are interrogated remotely to extract characteristics that can be used to determine blood pressure.
For example, the resonant frequency of an LC circuit of the sensor may be configured to vary as the pressure varies. By detecting changes in the resonant frequency, changes to blood pressure may be determined. As a result, a sensor reader should have a frequency band wide enough to detect frequencies within a range of interest. To detect changes in the resonant frequency, an external energy field is applied to excite the LC circuit of the sensor. When excited, the LC sensor emits a response signal, which is detected by the sensor reader. The sensor reader uses the response signal to determine the resonant frequency of the LC circuit. However, the intensity of the response signal received by the reader is relatively low, particularly when the sensor is placed deep inside the human body.
Several systems and methods for determining the resonant frequency of an implanted passive LC sensor have been discussed, including the following: U.S. Pat. No. 6,015,386, which excites the LC circuit by a frequency sweep of radio-frequency (RF) energy and then uses a phase detector to locate the resonant frequency; U.S. Pat. No. 7,245,117, which excites the LC circuit by a burst of RF energy at a predetermined frequency or set of frequencies and uses a phased-locked-loop (PLL) circuit to lock onto the sensor's resonant frequency; and U.S. Pat. No. 8,432,265, which discusses an improved reader system using a PLL circuit.
In general, to determine a resonant frequency with the frequency-sweep or phase-lock-loop techniques discussed in these references, the reader may have to fire radio-frequency (RF) excitation pulses many times at a set of predefined frequencies that includes the resonant frequency of the targeted LC circuit; therefore, a wide-band RF signal generator is needed to generate the frequency range of interest. Each excitation pulse may be a sinusoidal burst at a fixed frequency. At the same time, a wide-band RF power amplifier is needed to amplify each fired pulse at each predefined frequency in order to achieve a good signal-to-noise ratio (SNR) in the measurement. However, energy fired at frequencies away from the targeted resonance is wasted. Moreover, wide-band, high-power RF amplifiers are not energy-efficient, often requiring a heat-sink and fan to dissipate heat. As a result, these sensor readers are expensive and bulky. Furthermore, in the case of a sensor with multiple LC circuits, resonant frequencies from each of the LC circuits must be read. A single PLL circuit, however, cannot read multiple frequencies simultaneously. Thus, multiple PLL circuits are required with those sensor readers to simultaneously read multiple frequencies. This results in complex readers that are both large and expensive.
In some simplified reader systems, which seek to simplify reader circuitry and control software/hardware as well as address power consumption, the reader may be designed to fire an RF pulse only at a fixed center frequency (with a limited frequency bandwidth) at or near the center of the sensor's operating frequency range. However, the limited frequency bandwidth of the fixed frequency must cover the sensor's operating frequency range; otherwise, some of the sensor's frequency responses may be out of the reader's measurement range. The wider the sensor's operating frequency range, the wider the reader's frequency bandwidth must be. At the end, a relatively wide-band RF amplifier may be required, which is not energy efficient, as discussed above. If a reader is configured with a limited frequency bandwidth at a fixed center frequency, the reader doesn't have a uniform SNR over the sensor's operating frequency range; the highest SNR occurs only when the sensor's resonant frequency is at the reader's fixed center frequency, and the SNR decreases as the sensor's resonant frequency moves away from the reader's fixed center frequency. Moreover, for a pressure sensor with a wide pressure response range, if a reader is configured with a narrow frequency bandwidth at a fixed center frequency, the sensitivity of the sensor (i.e., the frequency change vs. the pressure change) may have to be decreased for the reader to cover its frequency range. As a result, measurement error may increase with decreased sensitivity. Further, a reader only firing at a fixed center frequency may not be able to simultaneously read multiple different resonant frequencies of a sensor with multiple LC circuits, in which different resonant frequencies must be separated.
Accordingly, a need exists for an efficient, wide-band, and compact sensor reader that improves energy efficiency by operating without a high power RF amplifier and/or reads a wide range of resonant frequencies and multiple resonant frequencies simultaneously, without requiring a wide-band signal generator. These readers could be more compact, energy efficient, and cost-effective.
As discussed below, several embodiments of the present disclosure address some or all of these issues as well as providing additional advantages.